feat(infospace): flat canonical entity set with cross-chapter deduplication

Restructure entity storage from per-chapter subdirectories to a flat
canonical set in output/entities/. Each entity exists as a single file;
duplicates across chapters are detected by slug collision and skipped
(first occurrence wins). Chapter views use {{ include }} transclusion
to reference shared entity files.

Add @{existing_entities} macro to extract-entities template so the LLM
knows which entities already exist and focuses on genuinely new ones.
Refactor _call_llm() from _execute_llm() for callers that handle their
own file I/O. 41 unique entities from 4 chapters (2 duplicates removed).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

examples/infospace-with-history/output/entities/porter.md

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+# Porter
+
+## Definition
+
+An urban labourer whose occupation consists of carrying goods and burdens for hire. Smith uses the porter as the exemplary case of a trade so specialised and dependent on volume of demand that it can only exist in a great town. A village or even an ordinary market-town cannot generate enough demand for carrying services to provide a porter with constant employment, making this trade the paradigmatic illustration of market-size-dependent specialisation.
+
+## Source Chapter
+
+Book 1, Chapter 3: "That the Division of Labour is Limited by the Extent of the Market"
+
+## Context
+
+The porter is introduced immediately after the chapter's thesis statement as the first concrete illustration. Smith notes that "a porter can find employment and subsistence in no other place" than a great town, establishing the principle that some trades require a minimum threshold of market activity to exist.
+
+## Economic Domain
+
+Production
+
+## Smith's Original Wording
+
+> "A porter, for example, can find employment and subsistence in no other place. A village is by much too narrow a sphere for him; even an ordinary market-town is scarce large enough to afford him constant occupation."
+
+## Modern Interpretation
+
+This anticipates the concept of minimum efficient scale and threshold effects in urban economics. Certain service occupations require minimum population densities to be viable — an insight formalised in central place theory (Christaller, 1933).